The present invention relates to a method for producing semiconductor bodies having a glass-covered defined edge profile.
It is known that in semiconductor devices a reverse voltage carrying capability of 1000 volts or more can be attained by making the jacket surface of a preferably disc-shaped semiconductor body oblique, at least in the region where the pn-junction or junctions come to the surface. With this type of configuration, which hereinafter will be called a facet configuration, the critical field intensity at the surface is raised beyond that in the interior and thus the excess voltage carrying capability is improved. In addition, at least the surface section in the region of the pn-junction is provided with a coating of an insulating and stabilizing protective lacquer so that the reverse voltage carrying capability obtained with the aid of such a facet configuration will not be adversely influenced by undesirable impurities on the semiconductor surface.
Semiconductor bodies with facet configuration have been produced by a method in which a basic semiconductor wafer having large-area, layer-type zones of different conductivity types is provided with metal contact layers for connection to current leads. The wafer is then divided into elements of smaller areal expanse, whereupon facets are produced on the elements by individual treatment including etching or grinding after which they are covered with a protective lacquer.
Such work on individual elements is particularly expensive and often leads to undue divergences in the characteristics of the intended semiconductor devices.
These drawbacks are avoided with the process disclosed in German Offenlegungsschrift No. 26 32 348. It relates to the production of semiconductor bodies with defined edge profiles which are produced by etching. A basic semiconductor wafer is provided on one side with a contact electrode and on the other side with a continuous, highly doped, external zone, and on the latter with contact electrode regions whose geometry is determined by the desired subdivision of the wafer. The wafer is further covered on both sides with an etch-resistant coating which is provided, on the side of the contact electrode regions, with openings for etching in a pattern corresponding to the intended subdivision, following which it is etched. During this etching process, through-like recesses are produced which have greatly sloping side faces and simultaneously constitute separating grooves for the provided elements of smaller areal expanse. Subdivision of the wafer results in elements with a facetted edge profile is then covered with a protective lacquer. The profile etching is particularly enhanced in that a short-circuit is produced at a suitable point between the contact electrode and between the inner zone bordering at the highly doped outer zone.
Although this method permits the economic production of semiconductor bodies with the desired edge profile, it does not in all cases meet all of the particular requirements. Tests have shown that it is not always assured that the protective lacquer adheres sufficiently and uniformly enough or that the semiconductor surface is covered with the lacquer in a tight seal. Moreover, the protective lacquer often exhibits undesirable reactions with substances contained in the encapsulation masses that sealingly encase the semiconductor bodies.
These drawbacks can be overcome by providing glass-like coatings on the semiconductor surface of the edge profile. However, the use of such coatings in the above-mentioned process results in untenably high expenditures. Such glass coatings are produced at about 800.degree. C. in an oxidixing atmosphere. At such process temperatures, however, the contact metal layer applied before the basic semiconductor wafer was etched also oxidizes and thus becomes unfit for use. The first contact metal coating must therefore be removed after the etching step and must be renewed after the glass coating has been applied. There then exists the additional danger that metal will remain on the semiconductor surface and adversely affect the efficiency of the surface treatment by means of the glass cover. Thus, in the known procedure, the edge profile, to be economical, can be coated only with a protective lacquer.
There also is known a process for producing semiconductor bodies with stabilizing glass coverings in which the basic semiconductor wafer is provided on its one or on both sides with trough-like recesses in addition to the separating grooves and in parallel therewith, and the faces of these recesses are covered with a glass coating. In such structures, the active surface area for current carrying capability is substantially reduced.
It is an object of the invention to make more economical the production of semiconductor bodies having a defined edge profile which are produced by etching and are covered with a glass coating, and to obtain semiconductor bodies that have an optimum amount of active surface area.